Method and apparatus for measuring fetal heart rate and an electroacoustic sensor for receiving fetal heart sounds

ABSTRACT

Method and apparatus for long-term, non-invasive measuring of fetal heart rate. The method utilizes the characteristic curves of first and second heart sound received by the preferred electroacoustical converter. The identification and distinction of first and second sounds, furthermore their time relation used for heartbeat identification highly increase the reliability of fetal heart rate determination. The distinction of first and second sound is based on the differences in frequency spectrums measured in a relatively short time window and by the estimation of the power peaks measured on two test frequencies chosen on the two ends of the frequency range of fetal heart sound. The method is implemented in an integrated apparatus to achieve low power consumption for battery-operated long-term measurements. Digital filtering and selective power estimation is applied for continuous computation of power time function on the two test frequencies. The test frequencies can be adaptively modified depending on individual fetal parameters to improve the distinction of the first and second sound. The apparatus is capable to registrate, store and transfer data of fetal heart rate and womb contractions to a personal computer.

TECHNICAL FIELD

This invention relates to particularities of monitoring the well-beingof a fetus on the basis of fetal heart activity.

BACKGROUND OF THE INVENTION

The widely used CTG equipment for monitoring of fetal heart activityapply ultrasound Doppler technique, where an ultrasonic beam is directedto the fetus, the reflection of which determines the heart movement andhereby fetal heart rate (FHR). However, these equipment are unsuitablefor long-time monitoring without inspection, since uncontrolledhigh-dose exposure may have some harmful effects.

A further technique for this purpose is the phonocardiography (PCG)where acoustic waves excited by heart movement are detected. In the caseof fetal heart sounds, however, difficulties arise in the detection andidentification of the signals. Acoustic waves of fetal origin reach thesensor placed on the maternal abdomen through a complex transmissionpath, where significant spectrum variation occurs. Furthermore,disturbances of maternal digestive organs may hinder signal detection.Finally, fetus movements result in the displacement of the optimalsensing point and lead to the decrease of signal level.

An essential advantage of the acoustic method is that the passive mannerof the sensing is harmless to the fetus even at very long monitoringtime, which makes possible home care. The detection and processing ofacoustic signals of fetal heart are dealt with by many researchers.

A fetal monitor is announced in U.S. Pat. No. 2,536,527 to Appel. Theinvention serves for monitoring fetal condition during delivery. Amicrophone applied to the stethoscope produces a signal which isamplified, filtered, rectified and used to indicate abnormally high orlow FHRs.

U.S. Pat. No. 3,187,098 to Farrar describes a fetal heartbeat detector,which uses a cantilevered piezoelectric crystal mounted within acontacting slab. A fetal monitor is given in U.S. Pat. No. 3,409,737 toSettler et al. This monitor is used with a belt having threemicrophones. An amplifier is used to selectively amplify the fetalheartbeat and remove the maternal heartbeat.

U.S. Pat. No. 4,781,200 to Baker uses a sensor belt wearing twelvesensors, the detected signal of which are compared to canceldisturbances and trace fetal position. The signal processing is carriedout by the conventional FFT method for 128 points applied in every ⅛seconds, delivering the frequency spectra for selection of coincidedcomponents of the sensors. However, because of the relatively long timeperiod of analysis the fast variations in spectral power density areaveraged. Thus short time characteristics of frequency components cannotbe correctly detected using this method. U.S. Pat. No. 5,140,992 toZuckerwar et al. uses a belt wearing more piezoelectric polymer filmsensors for fetal heartbeat indication.

All of this invention have the common insufficiency that they do notdistinguish perfectly the first and second sound and thus they cannotapply this additional information to the identification of fetalheartbeat sound. In addition, a lot of computations are carried out insignal processing, which can be saved without the deterioration of thereliable sound identification. As a consequence, these instrumentsrequire high supply current and thus they are for battery operationunsuitable.

Reliable identification of fetal heartbeat is of vital importance at FHRmeasurement. Consequently, there is need for a solution that enablesreliable identification of fetal heartbeats, does not require bulkyhardware and allows battery operation even without medical supervision.

GENERAL DISCLOSURE OF THE INVENTION

In a first aspect the aim of the invention is to provide a method foridentifying fetal heart sounds with improved reliability. According to asecond aspect, the method should be implemented on a low-powerelectronic circuit to allow designing a portable apparatus suitable forlong-term home monitoring. According to a further aspect, the apparatusshould enable easy finding of the correct position of the sensor on thematernal abdomen. Advantageously, simultaneous measurement of maternalwomb contractions should be possible by using a conventionaltocodynameter. According to a further aim of the invention a sensorshould be applied that enables improved acoustical detection of fetalheart activity.

The above and other aims can be achieved by the invented method formeasuring fetal heart rate on the basis of acoustic signals detected atthe maternal abdomen. The invented method comprises the following steps:

converting said acoustic signals into electric signals;

determining power levels P_(a)(t) and P_(b)(t) of said signals offluctuating intensity at test frequencies f_(a) and f_(b) being in alower and higher frequency band of the range of 20 to 80 Hz;

detecting and storing local peak values P_(a1) and P_(a2); as well asP_(b1) and P_(b2) of said power level and relevant times t_(a1) andt_(a2); as well as t_(b1) and t_(b2) for an inspection period t_(s)following the first one of said detected local peaks;

counting numbers n_(a) and n_(b) of said peaks of power levels duringsaid inspection period t_(s);

storing the value of said power level P_(b)(t_(a1)) when a peak valueP_(a1) at said lower test frequency f₁ is detected;

determining time differences dt=t_(a1)−t_(b1) and dt=t_(a2)−t_(b2)between detection of first peak values P_(a1) and P_(b1), or P_(a2) andP_(b2) within said inspection period;

classifying detected peaks on the basis of numbers n_(a), n_(b), valuesand timing of said peaks of power levels, and identifying peaks meetingpreset criteria as first and second sounds, respectively;

measuring the time difference between an identified first sound and asubsequent second sound representing the closing time;

identifying said first and second peak as a matching pair of sounds of aheartbeat if said closing time is in the range of 140 to 220 ms; and

calculating fetal heart rate from a time difference between twoconsecutively identified heartbeats.

The detected peaks are classified on the basis of the followingcriteria:

if one local peak is detected on power level P_(a)(t) as well on powerlevel P_(b)(t), that means n_(a)=1 and n_(b)=1, and for the timedifference dt=t_(a2)−t_(b2) between the two peaks the criterion −15ms<(t_(a2)−t_(b2))<15 ms is satisfied, then the detected peak is asecond sound (see FIG. 3a, right side),

if one local peak is detected on power level P_(a)(t) as well on powerlevel P_(b)(t), that means n_(a)=1 and n_(b)=1, and for the timedifference dt=t_(a1)−t_(b1) between the two peaks 15 ms<dt<40 ms,furthermore, for the power level P_(b)(t) theP_(b)(t_(a1))/P_(b)(t_(b1))<0.3 criterion is satisfied, then thedetected peak is a first sound (see FIG. 3a, left side),

if one local peak is detected on power level P_(a)(t) and two peaks onpower level P_(b)(t), that means n_(a)=1 and n_(b)=2, and for the peakvalues on the power level P_(b)(t) the P_(b)(t_(a1))/P_(b)(t_(b1))<0.3criterion is satisfied, then the detected peak is a first sound (seeFIG. 3b),

if two local peaks is detected on power level P_(a)(t) and one peak onpower level P_(b)(t), that means n_(a)=2 and n_(b)=1, and for timedifference dt=t_(a1)−t_(b1) between the two peaks the criterion −15ms<(t_(a2)−t_(b2))<15 ms is satisfied, then the detected peak is a firstsound (see FIG. 3c),

if two local peaks is detected on power level P_(a)(t) as well on powerlevel P_(b)(t), that means n_(a)=2 and n_(b)=2, then the detected peakis a first sound (see FIG. 3d),

Peaks not meeting criteria are classified as undefined sounds. This kindof classification enables identification of heart sounds with animproved reliability. Further criteria, especially for non-typical casescan also be of use. Preferably, said estimation of power P_(a)(t) atsaid lower test frequency fa comprises averaging for a time window of 30to 90 ms; said estimation of power level P_(b)(t) at said higher testfrequency f_(b) comprises averaging for a time window of 20 to 60 ms. Bythis measure generally more than one and less than three periods of thesound signal can be evaluated.

Local peak values P_(a1) and P_(a2); as well as P_(b1) and P_(b2) ofpower levels P_(a) and P_(b) are accepted preferably only if theirvalues are exceeding the {fraction (1/10)} part of the average ofpreviously detected peak values. Thereby noises and interferences haveless influence on the reliability of FHR values. Preferably, said lowertesting frequency is selected within range 25 to 35 Hz and said uppertesting frequency within range 55 to 65 Hz. Thereby a sufficientdistinction can be established between said first and second sounds.Other frequency ranges can also be involved to gain additionalinformation.

Test frequencies f_(a) and f_(b) can be predetermined by consideringweeks of gestation and estimated weight of the fetus, and these can beapplied as starting values at the beginning of the measurement. Theactual values of said test frequencies can be continuously adjusted onthe course of measurements to obtain maximum values for said local peaksof said signal power.

Generally, said inspection time t_(s) is longer than 80 ms and shorterthan 120 ms. However, said inspection time is shortened, ifidentification of said first sound or said second sound is completed.The closing time usually longer than 140 ms and shorter than 220 ms, butit can be continuously adjusted depending on previously measured values.

In a second aspect the invention relates to an apparatus forimplementing the above method. The invented apparatus comprises:

an acoustic sensor for sensing acoustic vibrations deriving from afetus, said sensor adapted to be placed onto maternal abdomen;

an analog unit for amplifying and enhancing frequencies characteristicto the fetal heart activity,

an amplifier having adjustable gain for adjusting the signal level to bein an evaluation range;

an analog to digital converter converting analogous signals to digitalsamples and connected to said amplifier;

a frequency selective power estimation unit for determining signal levelat a lower test frequency f_(a) within a frequency range from 25 to 35Hz;

a frequency selective power estimation unit for determining signal levelat a higher test frequency f_(b) within a frequency range from 55 to 65Hz;

a first and a second peak detector for monitoring said signal levelsfluctuating in time and estimated by said first and second powerestimation units and establishing occurrence of local peaks of saidsignal levels;

an identification unit for identifying first and second sounds basedupon time, values and sequence of said local peaks;

a computing means for computing fetal heart rate from said identifiedfirst and second sounds;

a storage means for storing computed fetal heart rate values;

a communication port for making said stored fetal heart rate valuesaccessible for further evaluation.

Said analog unit preferably comprises an active filter connected to saidsensor, said amplifier being connected to said active filter and a soundunit connected to said amplifier for amplifying, filtering and frequencytransponding of the input signal. Thereby low frequency signals of highamplitude can be filtered out in order to avoid overdrive of subsequentunits.

The apparatus can be designed to contain a conventional microcontrollercomprising analog to digital converters, memories, input/output portsand different dedicated units. Said microcontroller can be used forsound identification or for realization of said frequency selectivepower estimation units and said peak detectors.

According to another possibility, at least one of said frequencyselective power estimation units and said peak detectors are comprisedin an application specific processor that is connected to saidmicrocontroller. Said application specific processor can be realizedconventionally or in integrated form on a silicon chip. This applicationspecific processor can comprise also a gain control circuit connected tothe gain control input of said amplifier. The integrated applicationspecific processor allows substantial reduction of supply current andprolonged battery operation.

The apparatus can be completed with a tocodynameter for continuousmeasurement of womb contractions. Particularly for battery operation theapparatus should comprise a non-volatile memory connected saidmicrocontroller for storing measured fetal heart rate values andadditional data provided by said tocodynameter. Thereby these data willremain even if the battery is discharged.

To display of measured data and allow further evaluation the apparatuspreferably comprises a serial communication port connected to themicrocontroller for connecting said apparatus to a computer via astandard serial communication line.

To indicate subjective feeling of fetal movement and other additionalevents the apparatus can further comprise an input device, e.g. in formof several pushbuttons that are connected to said microcontroller. Toindicate amplifier overdrive and extremely low input signal level theapparatus can comprise light emitting diodes connected to saidmicrocontroller.

Said frequency selective power estimation units can be designed tocomprise a digital filter selecting frequencies within said frequencyrange and a power estimation unit connected to the output of saiddigital filter. Said digital filter advantageously comprises

a register file having serial-in, parallel-out structure and containingthe actual input data and number M preceding data to be considered atdigital filtering;

a segmented decoder consisting of decoder segments for separatelydecoding parallel outputs of said register file;

a segmented look-up table storing words to be designated separately bysaid segmented decoder consisting products a_(k)d_(n−k), where n is thenumber of sampling, d_(n−k) is the (n−k)th sampled input data, a_(k) arefilter coefficients with k being in the range of k=0 to M, where M isthe order of the filter;

a register file having serial-in, parallel-out structure and containingnumber N words of filters data;

a second segmented decoder consisting of decoder segments for separatelydecoding parallel outputs of said register file;

a second segmented look-up table storing words to be designatedseparately by said segmented decoder consisting products b_(k)y_(n−k),where y_(n−k) are (n−k)th previous filtered data, b_(k) are filtercoefficients with k being in the range of k=1 to N, where N is the orderof the filter, and the meaning of other designations are the same asabove;

a summator for consecutively reading out and summing each designed wordof said look-up table segments according to the following formula:${y_{n} = {{\sum\limits_{k = 0}^{M}{a_{k}d_{n - k}}} + {\sum\limits_{k = 1}^{N}{b_{k}y_{n - k}}}}},$

wherein y_(n) value is the actual filtered data, and the meaning ofother designations are the same as above. This structure of the digitalfilter requires minimal supply current.

In a preferred form said power estimator comprises

a decoder decoding filtered data as an address;

a look-up table addressed by said decoder and containing square valuesof said addresses;

a summator connected to said look-up table and summing valuesconsecutively read out of said table;

a scanner controlling repetitive and consecutive addressing and summingsaid square values by said summator;

a second register file connected to said summator and having serial-in,parallel-out structure and containing new summed data in its upperregister and oldest in its lowest register;

a replacer having a first input connected to the upper register of saidsecond register file a second input connected to lowest register ofregister file a third input connected to a third register file, saidreplacer adapted to adding actual value at its first input to andsubtracting actual value at its second input from the value receivedfrom said third register file;

a third register file connected to an output of said replacer and havinga serial-in, parallel-out structure storing calculated power values andshifting its contents down every step and having parallel outputs, thefirst of them connected to said third input of said replacer and allconnected to a peak detector and adapted to transfer contents to saidmicrocontroller. Said scanner controls power averaging for more steps.This structure minimizes supply current.

Second sounds can be evaluated more advantageously by a frequencyselective power estimator unit having another structure. Accordingly,said second frequency selective power estimator unit comprises

a register file having a serial-in, parallel-out structure and storinginput data d_(i) measured in a time window;

a decoder decoding data d_(i) stored in register file and designatingcorresponding words of a look-up table;

a look-up table containing products according to the following formulae:

V _(i) =d _(i)*sin(2πf _(b) i/n _(wb))*Ba(i);

W _(i) =d _(i)*cos(2πf _(b) i/n _(wb))*Ba(i);

where d_(i) is the ith data sampled and digitized from the unfilteredsignal, n_(wb) is the size of window in number of sample times, andBa(i) stands for the triangular correction function of Bartlett window;

a look-up table connected to summators and containing squared values aswords having a length equal to the word length of said summators;

a summator summing words of said look-up table designated by saiddecoder to provide an intermediate sum${S_{v} = {\sum\limits_{i = 0}^{n_{wb}}V_{i}}};$

where S_(v) is an intermediate sum, and squaring this sum by usinglook-up table for providing square intermediate sum of S_(v) ²;

a further summator summing words of said look-up table designated bysaid decoder to provide a second intermediate sum${S_{w} = {\sum\limits_{i = 0}^{n_{wb}}W_{i}}};$

where S_(w) is a further intermediate sum, and squaring this sum byusing look-up table for providing squared intermediate sum of S_(w) ²;

an adder adding squared intermediate sums for providing a power value ata second test frequency P_(b)=(S_(v) ²+S_(w) ²)/n_(wb), were P_(b) isthe power in the actual time window for frequency f_(b);

a register file having a serial-in, parallel-out structure and storinglast five sums of adder;

a peak detector for indicating peak values of said power P_(b).

The above structure needs minimal supply current which can be furtherdecreased if said decoders and summator are adapted to be turned on insleep mode to reduce power consumption when no heartbeat is expected.

A preferred arrangement of said peak detector comprise comparatorsconnected to parallel outputs of a corresponding register file and anAND gate is connected to outputs of said comparators. The output of thisAND-gate indicates occurrences of local peaks of the corresponding powerlevel P_(a) or P_(b) by a logical high state.

If monitoring is carried out for a longer time the sound produced bysaid sound unit may be disturbing and requires extra supply current. Toavoid this a switch is provided for to turn off audible heart sounds.

If no personal computer is available a hardcopy of measured data can beproduced by simply connecting a printer to said serial communicationport that is adapted to be connected to a conventional line printer,Another way of accessing measured data to use a standardized modemadapted to transfer data for telemetric measurements. This modem islinked to microcontroller via said communication port.

According to said third aspect the invention relates to an acousticsensor for sensing acoustic vibrations deriving from a fetus, saidsensor adapted to be placed onto maternal abdomen. This sensor isadapted particularly for use with the apparatus according to the aboveapparatus and comprises a hollow body and an electroacoustic converterlocated inside said hollow body and having a sensitive membrane. Saidhollow body is partitioned by walls into a first, a second and a thirdchamber, Said first chamber is open at one side, and is coupled to saidsecond chamber via an opening. Said first chamber has a boring on itssidewall and communicating with the atmosphere. Said second chamber hasan upper partition wall formed by a sensitive membrane of anelectroacoustic converter. Said third chamber is communicating with theatmosphere through a boring formed in the sidewall of said hollow body.

In a preferred embodiment the sensor said electroacoustic converter isan electrodynamic microphone having an elastic membrane. To achievereliable contact between said acoustic sensor and the skin of maternalabdomen an elastic ring is provided at the periphery of said open sideof said first chamber.

The present invention provides a method to distinguish reliably firstand second sounds of fetal heartbeats and an apparatus carrying out thismethod. The distinction of the two sounds increases significantly thereliability of heartbeat identification and results in correct fetalheart rate values even at high level of disturbances. The method isimplemented in a low-power portable equipment, which allows long-timeFHR measurements and data registration. Low power consumption isachieved by the utilization of an application specific processoroptimized for minimum number of operations.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aims, advantages and features of the invention will be discussedwith reference to a preferred embodiment of the invention that isillustrated in the accompanying drawings. In the drawing:

FIG. 1 shows time function of a pair of sound burst caused by a fetalheartbeat.

FIG. 2 shows frequency spectra of the first and second sound.

FIG. 3 shows four typical time functions of heartbeat sound powerdensities calculated at frequencies f_(a)=30 Hz and f_(b)=60 Hz.

FIG. 4 is the flow-diagram of separation and identification of first andsecond sound of fetal heartbeat.

FIG. 5 shows block diagram of a preferred embodiment of the inventedapparatus adapted for continuous registration of fetal heart rate.

FIG. 6 is a cross-sectional view of preferred embodiment of the inventedsensor for use with the apparatus of FIG. 5.

FIG. 7 is a detailed block diagram of a preferred embodiment of thedigital filter circuit of FIG. 5.

FIG. 8 is a detailed block diagram of a preferred embodiment of thepower estimation circuit of FIG. 5.

FIG. 9 is a detailed block diagram of a preferred embodiment of theselective power estimation circuit according to FIG. 5.

FIG. 10 is a detailed block diagram of a preferred embodiment of thepeak detector circuit of FIGS. 8 and 9 for the indication of peak valuesP_(a)(t) and P_(b)(t).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows the time function of the substantially noiseless sound of afetal heartbeat, where the first sound is related to the contraction,the second one to the relaxation of the heart. Closing time means thetime interval between these sound. The signal shown in the figure doesnot contain components below 25 Hz that were high-pass filtered todepress the low-frequency disturbances.

The frequency spectra of the first and second sound averaged for a longmeasuring time are shown in FIG. 2. However, there may be short timeperiods, when the frequency components differ substantially from theaverage distribution. In a typical case the low-frequency componentsdisappear from the first sound. Because of the temporary variations ofthe spectrum, the curves of FIG. 2 are unusable for the reliabledistinction of the two wounds.

The invention is based on the results of a large number of measurementsshowing that here is a significant difference between the powerdensities of the first and the second sound measured in a short time.This phenomenon can be well indicated on the spectral distribution ofthe power density determined for a relatively short time window.Analyses on large number of fetuses in the range of 25-80 Hz have shown,that there are some characteristic features of the power-time functionsmeasure on frequencies of the upper and lower part of the range. Becausethe disturbing effect of the maternal heartbeat sounds, frequenciesunder 25 Hz are excluded from the analysis. Furthermore, componentsabove 80 Hz were neglected, since the power density falls there veryrapidly.

The following discussion is directed to a lower test frequency f_(a)being between 25 and 35 Hz and relevant power density P_(a)(t), and ahigher test frequency f_(b), being between 55 and 65 Hz, and relevantpower density P_(b)(t). Four typical signal curves of time functions ofthese components are shown in FIG. 3, where the P_(a)(t) and P_(b)(t)curves are computed for f_(a)=30 Hz and f_(b)=60 Hz and averaged fortime windows t_(wa)=60 ms and t_(wb)=40 ms assigned to said two testfrequencies. The length of these time windows should be longer than oneand shorter than three periods of the test frequency.

In FIG. 3/a both curves have one peak at the first sound and one at thesecond sound. As observed, in most of such cases the peak of P_(b)(t)precedes with 15-40 ms the peak of P_(a)(t), so the criterion 15ms<t_(a1)−t_(b1))<40 ms is fulfilled. In addition, for the value ofP_(b)(t) at t=t_(a1) and the peak value of P_(b)(t_(b1)) the criterionP_(b)(t_(a1))/P_(b)(t_(b1))<0.3 is satisfied where t_(b1) is the time ofthe first peak. On the other hand, at the second sound the two peaks arevery near together, i.e. the criterion −15 ms<(Error, the marker is nottrue t_(a2)−t_(b2))<15 ms is satisfied.

In FIG. 3/b P_(b)(t) has two peaks at the first sound. However, thecriterion of P_(b)(t_(a1))/P_(b)(t_(b1)) 0.3 is still satisfied. Thesecond sound is similar to that in FIG. 3/a. In FIG. 3/c P_(a)(t) hastwo, Pb(t) one peak at the first sound. In most of such cases for thetime of the first peak t_(a1) of the first peak P_(a)(t) the criterionof −15 ms<(t_(a1)−t_(b1))<15 ms is satisfied. In FIG. 3/d both P_(a)(t)and Pb(t) have two peaks at the first sound. In most of such cases thissound is a first one and no further criterion should be considered.

A large number of tests have proved that in most cases the overwhelmingpart of fetal acoustic heart signals is quite similar to one of thesecases, meets the conditions mentioned above, consequently the first andthe second sound can be reliably distinguished. A flow diagram of thedistinction of the sounds is shown in FIG. 4, where dt denotes the timedifference between the first peaks of P_(a)(t) and P_(b)(t), furthermoret_(1,2)=t_(a2)−t_(a1) is the time difference between the first andsecond sounds and, finally, the number of peaks of P_(a)(t) and P_(b)(t)detected during the inspection time interval of 80 to 120 ms, preferablyt_(s)=100 ms are denoted to n_(a) and n_(b), respectively.

As seen on the flow diagram, the method continuously searches for localpeaks of P_(a)(t) and P_(b)(t), which should exceed the {fraction(1/10)} part of the average of, e.g. five preceding peak values. Findingthe first acceptable peak the counting of the inspection time t_(s)begins. During this time all accepted local peaks are stored, countedand averaged. Ending the inspection time the number of peaks areexamined and sounds are classified considering the number, values andtiming of the peaks. Sounds can be classified as first sounds, secondsounds or undefined sounds. This classification allows identification offetal heart sounds with high reliability. The classification is executedon the basis of various criteria constituting a substantial aspect ofthe invented method and discussed in detail later.

In an identified first sound is followed by an identified second onewithin a subsequent closing time range between 140 and 220 ms, then thepair of sounds are identified as a heartbeat. The actual FHR value iscalculated from the reciprocal of the repetition time of two subsequentidentified heartbeats as FHR=60/T_(rep), where T_(rep) is therepetition; time. To improve identification reliability the duration ofsaid closing time range can be continuously adjusted depending on theaverage of previously measured closing times.

A first or a second sound detected separately, i.e. without itscorresponding pair can be utilized to substitute a not identifiableheartbeats. If the time difference of two identified heartbeats isapproximately twice as long as the previously calculated ones, i.e. 1.4to 2.6 multiple thereof, furthermore, a first or second sound was foundwithout a matching pair between two heartbeats, then the half of themeasured time difference can be considered for FHR calculation.

The theoretically exact estimation of power values needs numerousarithmetic operations that cannot be completed by a low-power electroniccircuit within a sampling period. To overcome this problem significantsimplifications are introduced in the computations and the word lengthis truncated, without sacrificing the distinction of the first andsecond sound. Since the power consumption of advanced CMOS circuits isproportional to the number of operations this number should besubstantially reduced. This can be accomplished by utilizing anapplication-specific processor.

An arrangement of the apparatus implementing the invented methoddiscussed above is shown in FIG. 5. Acoustic signals generated by afetus are converted into low-level electric signals by a speciallyconstructed sensor 1 placed to the surface of the maternal abdomen. Saidsensor 1 is connected to an analog unit 2. An active filter 3 comprisedin said analog unit 2 enhances signals in a frequency band from 25 to 80Hz. Filtering out frequency components below 25 Hz and above 80 Hzseparates signal components to be analyzed from the disturbancesconsisting of maternal heart sounds and external noises.

Said analog unit 2 further comprises an amplifier 4 having adjustedgain, said amplifier 4 amplifies the output signal of filter 3 to alevel required for subsequent analog to digital conversion. For thispurpose continuous gain control compensates level variations of thereceived signal. Said analog unit 4 comprises a sound unit 5 connectedto an output of said amplifier 4 and containing a power amplifier, afrequency transponder and a loudspeaker to make the input signalaudible. The frequency shift by, e.g., 150 Hz avoids instability of thepower amplifier and makes sound more perceptible. This transpondedaudible sound of heartbeat makes easier to find the optimal location ofsensor 1 on the maternal abdomen. Preferably a switch is provided for toswitch off this sound.

The output of amplifier 4 is fed to an analog to digital converter 61comprised in an advanced, low-power microcontroller 6. Said converter 61converts amplified signals with a sampling time of T=2 to 4 ms andforwards digitized data to an application specific processor 7 forestimating powers P_(a)(t) and P_(b)(t) and detecting their local peaks.Accordingly, said application specific processor 7 includes a gaincontrol unit 18, a digital filter 15, a first selective power estimationunit 16 connected to said digital filter 15 and a second selective powerestimator unit 17.

Said first selective power estimator unit 16 has a first outputproviding the actual value of P_(a)(t) and a second output providing asignal if a local maximum or peak of P_(a)(t) is detected. More detaileddescriptions of digital filter 15 and first selective power estimatorunit 16 are given later referring to FIG. 7 and FIG. 8, respectively.

Processor 7 further includes a second selective power estimation unit 17having a first output providing the actual value of P_(b)(t) and asecond output indicating peaks of P_(b)(t).

Microcontroller 6 performs arithmetic operations of the preferredalgorithm demonstrated by the flow diagram of FIG. 4. Microcontroller 6includes a number of circuitry well-known in the art and not mentionedseparately and allowing the microcontroller 6 to perform all functionsof an also separately realizable identification unit for identifying orqualifying sound bursts. Said qualification is based upon time, valueand sequence of local peak of P_(a)(t) and P_(b)(t), respectively, andpreformed as disclosed herein and demonstrated by FIG. 4.

Receiving a signal indicative of a peak said microcontroller 6 reads outthe actual values of P_(a)(t) and P_(b)(t), and evaluates these values.As a result of this process received sound bursts are qualified as afirst or a second or unidentified sounds, furthermore, a heartbeat willbe identified, when a corresponding pair of a first and a subsequentsecond sound is found. The reciprocal of the repetition time ofidentified heartbeats provides an actual FHR value, which iscontinuously stored in a non-volatile memory 8 connected to andcontrolled by said microcontroller 6. Stored data can be read out, e.g.by a personal computer through the microcontroller 6 using a standardserial line 10. The high-capacity non-volatile memory 8 is capable tostore the data of a measurement more than one week long, and makesthereby the apparatus suitable for long-term home monitoring. Thepersonal computer may be omitted by connecting a line printer directlyto serial line 10 forming in this way a cost-effective configuration forFHR printout.

A critical point of the invention is the accuracy of the digitizedinput. To achieve sufficient accuracy, full range of an 8-bit A/Dconversion should be exploited. To assure this, a gain control unit 18is included in processor 7 and is connected to microcontroller 6 and tothe gain control input of amplifier 4. The average value of detectedpeaks should be set equal approximately to the 75% of the full scale.The gain control unit 18 is regulated by said microcontroller 6, whichcontinuously calculates average values of the detected peaks of P_(a)(t)and P_(b)(t).

As a consequence of this adjustment the amplifier 4 or even the analogto digital converter 61 will be sometimes overdriven by very highdisturbances. However, these highly disturbed time periods can be leftout of consideration because they do not provide information. It isimportant however, that after them the system should recover to itsnormal operation as fast as possible.

The gain control unit 18 comprises a digital to analog converter, or canbe realized as a manually adjustable resistor. Since the operation isnot sensitive to gain accuracy, the resolution of digital-analogconverter in gain control unit 18 can be less than 5 bit.

Preferably the apparatus comprises some conventional units such as atocodynameter 9, an input device 12 in form of one or more pushbuttons,optionally light emitting diodes 13 and 14, all in connection with saidmicrocontroller 6. It controls the tocodynameter 9, which monitorscontinuously maternal womb contractions. Said input device 12 allows themother to indicate the subjective feeling of the fetal movement. Theoptional light emitting diode 13 indicates overdrive of the amplifier 4due to extremely large input acoustic signal, light emitting diode 14signalizes underdrive, when extremely low input level makes heartbeatidentification problematic. The serial line 10 realizes a standard linkbetween the microcontroller 6 and an external personal computer.

The structure of sensor 1 is shown in FIG. 6. This design providesmaximum sensitivity in the frequency band to be analyzed. Sensor 1comprises a hollow body 27 divided into a first, a second and a thirdchamber 19, 20 and 25, respectively. First chamber 19 is open at oneside and is coupled to second chamber 20 via an opening 2 formed in thepartition wall between said first chamber 19 and second chamber 20. Theconstruction of the external first chamber 19, the internal secondchamber 20, and the coupling between them serve to obtain advantageousfrequency characteristics. The lower side of the first chamber 19 isconfined by the skin of the maternal abdomen, whereas the sidewall isrigid. The first chamber 19 is acoustically closed by elastic ring 22provided at the periphery of first chamber 19. The surrounded part ofthe skin surface acts as a membrane. The volume of first chamber 19 is250 to 350 cm³. Ring 22 has an external diameter of 70 to 80 mm, itswidth is 5 to 6 mm and the pressure by which the sensor is touching theskin is 0.002 to 0.003 kp/cm². This pressure is accomplished by aflexible belt.

The upper wall of second chamber 20 is formed by a sensitive membrane ofan electroacoustic converter 24, whereas its sidewall is rigid. Thevolume of second chamber 20 is 130 to 200 cm³. This central boring 21between first chamber 19 and second chamber 20 realizes acousticcoupling of the chambers. The diameter of the boring 21 is 2.5 to 3 mm.The volume of first chamber 19 and second chamber 20, furthermore thesize of central boring 21 accomplish acoustic matching between theabdomen and the electroacoustic converter 24.

The electroacoustic converter 24 is a highly sensitive dynamic converterwith elastic membrane, however, other types with sufficient sensitivity,e.g. piezoelectric converters, can also be used. To avoid overdrive ofthe active filter 3 by low-frequency components of the maternal heartsound, it is suitable to terminate the electroacoustic converter 24 withhigh impedance, cutting off thereby low frequency components.

On the sidewall of first chamber 19 a first boring 23 of diameter 0.5 to3 mm preferably 0.8 mm is formed to avoid damping effect of the air masstransferring the vibration of the skin. Said first boring 23 forms ahigh-pass filter acting as damping for the maternal heart sounds. Saidthird chamber 25 is located on the opposite side of the membrane of saidelectroacoustic converter 24 and is communicating with the atmospherethrough a second boring 26 of diameter 0.5 to 3 mm preferably 0.8 mmformed in the sidewall of hollow body 27. Said second boring 26 betweenthe third chamber 25 and the free air reduces the effect of the airbagdeveloped behind the electroacoustic converter 24, and providesprotection against background noise by compensation.

To estimate the powers P_(a)(t) and P_(b)(t) the frequency components off_(a) and f_(b) should be selected from the 25-80 Hz passband. Theapplication of active filters is unsuitable for this purpose because ofthe large number of discrete components required for the highselectivity. More advanced are the recursive digital filters, the y_(n)output of which is given by the relationship${y_{n} = {{\sum\limits_{k = 0}^{M}{a_{k}d_{n - k}}} + {\sum\limits_{k = 1}^{N}{b_{k}y_{n - k}}}}},$

where n is the number of sampling, d(n−k) is the (N−k)th previouslysampled input data, y(n−k) is the (n−k)th previous filtered data, a_(k)and b_(k) are the filter coefficients, and M and N represents the ordersof the filter. As seen from the relationship, every sampling requiresM+N+1 number of multiplication, which results to high power consumptionof the arithmetic unit. The power consumption is reduced by the digitalfilter 15 shown in FIG. 7.

This exemplary digital filter 15 comprises a register file 28 having aserial-in, parallel-out structure. The m-bit long digitized input dataare loaded in the register file 28, which store (M+1) number of wordsand parallel output of each register provide an m-bit word. Preferredvalue of m is 8. Upper register of register file 28 holds actual data,the lower stores the oldest one. These outputs are connected to inputsof a segmented first decoder 29 comprising the same (M+1) number of m to2^(m) decoders or segments as the number of registers in register file28. Each segment has 2^(m) number of outputs which are connected to adesignating line in a corresponding segment of a memory containing datawords of the look-up first look-up table 30. Each designating linedesignates a corresponding word in the corresponding segment of firstlook-up table 30. Consequently, (M+1) number of data words aredesignated at the same time. The words of the segments, corresponding tothe first member of the above formula contain the product of the inputdata and the filter coefficients a_(k) (a₀ to a₆). The word length offirst look-up table 20 is truncated to 16 bit, whereby the size is2^(m)(M+1) words, i.e. 7×256=1792 words of 16 bit comprised in (M+1)=7segments. The first look-up table 30 includes an internal bus comprisinga corresponding number of, i.e. 16 lines. Each segment of first look-uptable 30 has an activating input allowing to transfer designated wordsone-by-one to this bus. The bus, i.e. the output of first look-up table30 is connected to the firs summator 34. The designated words of thesegments are read out sequentially and summed by the said first summator34. The sum is truncated to m bit. Coming new data to the filter, thecontents of the register file 28 is shifted down. As a result, the datawill now designate the next segment of the first look-up table 30.

Digital filter 15 comprises a further second register file 33, a furthersecond decoder 32 and a further look-up second look-up table 31 havingthe same structures and interconnections as outlined above. The onlydistinction is that input data are received from the output of saidfirst summator 34 and the number of segments is N.

Applying recursive type digital filter the preceding values of filtereddata are also used. The values are fed back to the second register file33, organized as a serial-in, parallel-out storing unit and containing Nwords by m-bits. The operation of second register file 33, seconddecoder 32 and look-up second look-up table 31 is similar to that ofregister file 28, first decoder 29 and first look-up table 30. The sizeof the second look-up table 31 is 2^(m) N=1536 words by 16 bit comprisedin N=6 segments. The designated words of the segments of first look-uptable 30 are read out sequentially and summed by the said first summator34. The sum is truncated to m-bit. After summation of the M+N+1 productsthe first summator 34 outputs filtered data. Thus the upper register ofsecond register file 33 contains the last filtered data.

A further advantage of digital filtering compared to analog technique isthat a sleep mode can be introduced between the heartbeats to suspendfiltering for a given time, thus decreasing the power consumption of thecircuit. Receiving a sleep signal corresponding circuits turn intopowder down mode and output data of the filter fall to zero. The sleepsignal is generated by the microcontroller 6 after identification of aheartbeat. This sleep mode is 80 to 140 ms, preferably 120 ms longcounted from the respective second sound, i.e. t_(a2). During this sleepperiod no heartbeat is expected.

Using look-up tables with preloaded products of multiplication reduceshighly the power consumption of the circuit. The tables may be EPROMmemories with burned-in values, providing the lowest power requirementand the fastest operation. However, in this case the filtercharacteristics cannot be modified. To allow modification of thecharacteristics, the table should be stored in read-write memories. Thememory will be loaded by the microcontroller 6 depending on theevaluation of the power functions. This allows modifying filtercharacteristics while carrying out measurement depending on the spectrumof acoustic signals received from the fetus. Modification can beeffected by experienced personnel or an appropriate software can beprovided for. In addition, the filter coefficients can be calculated bya personal computer considering the progress of gestation and theestimated weight of the fetus in advance. These values can be loaded inthe microcontroller 6 as starting data.

The estimation of the power P_(b)(t) measured at the frequency f_(b)and, the detection of its peak values is carried out by theapplication-specific processor 7. For signals with zero mean the powerof n_(w) sampled data can be estimated by the relationship${P = {\sum\limits_{i = 0}^{n_{w} - 1}d_{i}^{2}}},$

where d_(i) is the ith sampled data and P is the summed power for thenumber n_(w) sampling. As seen, to calculate the power in every samplingcycle it is needed to store and sum the square of number n_(w) data inevery cycle, corresponding to the length of the mowing window. The largenumber of data to be stored increases unsuitably the required memorycapacity, furthermore, the large number of arithmetic operationsenhances the power consumption.

Since the sampling frequency 1/T is much higher, than the highestcomponent in the frequency band to be analyzed, some basic reductionsmay be introduced in the relationship above. The time function of thepower will be averaged for j cycles, furthermore, the preceding valuewill be utilized for calculation of the actual power value correspondingthe relation${P_{n} = {{P_{n - j} + {\sum\limits_{i = {n - j}}^{n}d_{i}^{2}} - {\sum\limits_{i = {n - n_{w} - j}}^{n - n_{w}}d_{i}^{2}}} = {P_{n - j} + P_{i\quad n} - P_{out}}}},$

where P_(n) is the calculated power in the nth cycle, P_(n−j) is thepower calculated before j cycles and n_(w) is the size of the windowexpressed in sample numbers, which should be a multiple of j. Inaddition, P_(in) is the power entering as the window moves forward andP_(out) is the power to be subtract. Using this simplified relationshipthe number of summation of the squared samples are reduced by j,furthermore, the number of sums to be stored is only n_(w)/j.

The number of computation can be further reduced, if the squaring isaccomplished by the utilization of a preloaded table. The schematic ofthe first selective power estimator unit 16 as one solution of thecomputation is shown in FIG. 8.

The first selective power estimator unit 16 can be advantageouslydesigned to include an m to 2^(m) third decoder 35 the outputs of whichare connected to the third look-up table 36. Output of the third look-uptable 36 is connected to a summator interconnected with a scanner 38.

The digitally filtered m-bit data control the m to 2^(m) third decoder35, which decodes an address. The address designates a word of the thirdlook-up table 36, which holds the square value of the address truncatedto 16 bit. The size of the third look-up table 36 is 2^(m) words by 16bit. The second summator 37 reads out and sum j-times consecutively thedesignated words, corresponding to the P_(in) member of therelationship. The j consecutive steps are controlled by the scanner 38.The power value P_(in) is loaded into the third register file 39, inwhich the data are shifted down in every j step. The upper register ofthird register file 39 holds now the new data P_(in) and the lowestregister contains the leaving data P_(out). Both registers are parallelconnected to the replacer 40, which substitutes P_(out) by P_(in)subtracting it from the previous value P_(n−j) and adding to them thenew one, corresponding to the relationship. The previous value P_(n−j)is read out from the upper register of fourth register file 41. Thecalculated new value P_(n) is loaded into the upper register of thefourth register file 41, in which the data are shifted down in every jstep. The data are readable by the microcontroller 6. The fourthregister file 41 contains five 16 bits words which are connectedparallel to the first peak detector 42.

The first peak detector 42 serves for the announcement of the maximumvalue of the power P_(a)(t). The circuit schematic is shown in FIG. 9.The adjacent pairs of the sequentially following P_(n), P_(n−1),P_(n−2), P_(n−3), and P_(n−4) values are compared by the comparators 43,44, 45, and 46, respectively. Comparison criteria are as follows:P_(n)<P_(n−1); P_(n−1) P_(n−2); P_(n−2)>P_(n−3); P_(n−3)>P_(n−4). As itfollows from the above criteria comparator 44 provides also logic highstate when the amplitudes are equal in order to take into account thecase when two equal maximum values are detected. When all conditions aresatisfied, the value P_(n−2) represents the peak of the power functionand the AND gate 47 transmits a signal to the microcontroller 6. Themicrocontroller 6 checks the amplitude of the peak and rejects them, ifthe amplitude is smaller than the {fraction (1/10)} part of the averageamplitude of the previously registered peaks.

To keep power consumption low the microcontroller 6 turns over firstpeak detector 42 in sleep mode, whenever a heartbeat is identified andso no beat is expected for the coming 120 ms time interval.

The power P_(b)(t) of the frequency component f_(b) is estimated also bythe application-specific processor 7 using the moving-window periodogrammethod by the relationship${S_{v} = {\sum\limits_{i = 0}^{n_{wb}}V_{i}}};{V_{i} = {d_{i}*{\sin \left( {2\pi \quad f_{b}{i/n_{wb}}} \right)}*{{Ba}(i)}}};$${S_{w} = {\sum\limits_{i = 0}^{n_{wb}}W_{i}}};{W_{i} = {d_{i}*{\cos \left( {2\pi \quad f_{b}{i/n_{wb}}} \right)}*{{Ba}(i)}}};$

where d_(i) is the ith data sampled and digitized from the unfilteredsignal, n_(wb) is the size of window in number of sample times, Ba(i)stands for the triangular correction function of the Bartlett window,and P_(b)=(S_(v) ²+S_(w) ²)/n_(wb) is the power in the actual timewindow for the frequency f_(b).

The schematic diagram of the second selective power estimator 17 forcarrying out the above computations is shown in FIG. 10. This preferredembodiment of the second selective power estimator unit 17 comprises afifth register file 48 having a serial-in, parallel-out structure. Fifthregister file 48 stores the last n_(wb) number of data and shifts themdown in each step. The digitized, m-bit input data d_(i) containing allfrequency components between 25-80 Hz are loaded in the fifth registerfile 48. The upper register holds actual data, the lower stores theoldest one. Each register has a parallel output connected to a segmentedm to 2^(m) fourth decoder 49. Each parallel output of fifth register 48controls one segment of an m to 2^(m) fourth decoder 49. Each segmentdecodes its input data and designates two corresponding words stored ina segmented fourth look-up table 50, which stores V_(i) and W_(i)products of the above formula, truncated to 16-bit. Said fourth look-uptable 50 can comprise two tables similar to look-up tables 30 and 31 ofdigital filter 15. Consequently, the size of the fourth look-up table 50is 2^(m)*n_(wb) words, divided into n_(wb) segments. The fourth look-uptable 50 is connected to the third and fourth summators 51, 52. Thedesignated products of V_(i) are summed by said third summator 51,whereas the designated product of W_(i) by said fourth summator 52. Thesummation is carried out n_(wb) times consecutively for all segments,corresponding to the above formulae. Summators 51 and 52 areinterconnected with fifth look-up table 53 comprising squared values ofinput data. On the basis of summed products, said third summator 51 aswell as said fourth summator 52 read out relevant squared values fromfifth look-up table 53 and transfers the squared values to an adder 54connected to the output of third and fourth summators 51, 52.

The squared values transferred by said third summator 51 and said fourthsummator 52 are added by adder 54 according to the above formula. Thisvalue will be loaded in a sixth register file 55 connected to the outputof said adder 54. Said sixth register file 55 has a serial-in,parallel-out structure and stores the last five values of the powerP_(b). Parallel outputs are connected to a second peak detector 56,which has the same structure as first peak detector 42 discussed above.Comparing amplitudes of the last five power values an incoming peak canbe detected and indicated to the microcontroller 6.

The size of fourth look-up table 50 can be reduced by rounding theproduct values. In this case, the least significant bits are cut downtruncating hereby the word length to m*<m. Preferred value of m* is 6 or7. As a result, there are more d_(i) input data assigned to the sameproducts, which can be drawn together in a common table segmentcontaining 2^(m*) words. This common segment can be addressed by ORcombining outputs of related decoder segments. In this way the size oftable can be significantly reduced without sacrificing accuracy ofP_(b)(t).

After identification of each heartbeat the microcontroller 6 turns overthe second selective power estimator unit 17 into sleep mode for 120 mstime interval to keep average power consumption low.

The reliability of heartbeat identification can be increased by theapplication of a second sensor placed to a location where the detectionof the maternal heartbeat sounds is maximal. To identify maternalheartbeats is very easy because of its large amplitudes. Utilizing theseidentified signals to inhibit fetal sound measurement temporary, thedisturbing effect of the maternal sounds can be reduced.

The program running on the person computer reads out the FHR andtocodynameter data from the non-volatile memory 8. Furthermore, it readsout data relating to the patient and the time of measurement. Auser-friendly paging software serves for the survey of large datablocks.

For telemetric home care applications the equipment can be completedwith a modem to send data into the hospital or to the doctor's privatepersonal computer. The equipment can be provided with a handler routinefor direct driving of a printer thus eliminating the need for a personalcomputer, when a common printout is required only.

The application specific processor 7 is implemented in a low-power CMOScircuit. In order to minimize power consumption it is designed forminimum internal parasitic capacity to be charged during one cycle ofsampling. If higher battery current is allowed, then some operationsperformed by the application specific processor 7 can be carried out bythe microcontroller 6 instead. The evaluation of the detected peaks bythe microcontroller 6 takes only some percentage of time thus relevantpower consumption is negligible.

What is claimed is:
 1. A method for measurement of fetal heart rate onthe basis of acoustic signals originating from heat activity of a fetusand detectable on a maternal abdomen, the method comprising the stepsof: measuring acoustic signals of fluctuating intensity originating fromthe heart activity of the fetus; converting said acoustic signals intoelectric signals; determining power levels (P_(a), P_(b)) of saidsignals at a lower and at a higher test frequency (f_(a),f_(b)), saidlower and higher frequencies (f_(a),f_(b)) being, respectively, in alower and a higher frequency band of a frequency range of 20 to 80 Hz;detecting and storing local peak values (P_(a1), P_(a2); P_(b1), P_(b2))of said power levels (P_(a), P_(b)) at different times (t_(a1), t_(a2);t_(b1), t_(b2)) during an inspection period (t_(s)) following the firstone of said detected local peaks; counting numbers (n_(a), n_(b)) ofsaid peaks of power levels during said inspection period t_(s);determining time differences between the times of detection of said peakvalues (P_(a1), P_(b1), P_(a2), P_(b2)) within said inspection period(t_(s)); identifying the detected peak as a said second sound, providedthat said number (n_(a)) of local peaks of said power level (P_(a)) isone and said number (n_(b)) of local peaks of said power level (P_(b))is one and said time difference (dt) between said peak values (P_(a2),P_(b2)), is smaller than 15 ms; identifying the detected peak as a saidfirst sound, provided that said number (n_(a)) is one and for said timedifference (dt) the criterion 15 ms<dt<40 ms is satisfied and for saidpower level (P_(b)) measured at said time (t_(a1)) corresponding of saidpeak values (P_(a1)) of said power level (P_(a)) the criterion(P_(b)(t_(a1)))/P_(b1)<0.3 is satisfied; identifying the detected peakas a said first sound provided that said number (n_(a)) is one and saidnumber (n_(b)) is two and for said power level (P_(b)) measured at thetime (t_(a1)) of said peak values (P_(a1)) of said power level (P_(a))the criterion (P_(b)(t_(a1)))/P_(b1)<0.3 is satisfied; identifying thedetected peak as a said first sound, provided that said number (n_(a))is two and said number (n_(b)) is two; classifying a pair of identifiedpeaks as heartbeat, provided that the measured time difference (t_(1,2))between the said first and second sound is in the range of 140 ms to 220ms; and calculating the fetal heart rate from the time differencebetween two consecutively identified heartbeats.
 2. The method asclaimed in claim 1 wherein said determination of power level (P_(a)) atsaid lower test frequency (f_(a)) includes averaging said power level(P_(a)) for a time window of 30 to 90 ms; and said determination ofpower level (P_(b)) at said higher test frequency (f_(b)) includesaveraging said power level (P_(b)) for a time window of 20 to 60 ms. 3.The method as claimed in claim 1 or 2, wherein local peak values(P_(a1), P_(a2); P_(b1), P_(b2)) of power levels (P_(a), P_(b)) aredetected if their values exceed the {fraction (1/10)} part of theaverage of previously detected peak values.
 4. The method as claimed inclaim 1, wherein said lower testing frequency band ranges from 25 to 35Hz and said upper testing frequency band ranges from 55 to 65 Hz.
 5. Themethod as claimed in claim 1 wherein said testing frequencies(f_(a),f_(b)) are predetermined according to the time of gestation andestimated weight of fetus, and said predetermined test frequencies areapplied as starting values at the beginning of the measurement.
 6. Themethod as claimed in claim 1 wherein the values of said testingfrequencies (f_(a),f_(b)) are continuously adjusted to obtain maximumvalues for said local peaks of said signal power.
 7. The method asclaimed in claim 1 wherein said inspection time (t_(s)) is longer than80 ms and shorter than 120 ms.
 8. The method as claimed in claim 1wherein said inspection time (t_(s)) is shortened if identification ofsaid first sound or said second sound is completed.
 9. The method asclaimed claim 1 wherein the time difference (t_(1,2)) between the firstand second sound is longer than 140 ms and shorter than 220 ms. and saidtime difference (t_(1,2)) is continuously adjusted on a previouslycalculated average of the time difference (t_(1,2)).
 10. An apparatusfor measuring fetal heart rate, the apparatus comprising: an acousticsensor (1) for sensing acoustic vibrations derived from a fetus, saidsensor adapted to be placed on a maternal abdomen; an analog unit (2)for amplifying and enhancing frequencies characteristic to fetal heartactivity, the analog unit (2) including an amplifier (4) havingadjustable gain for adjusting a signal level to be in a conversion rangeof an analog to digital converter (61) connected to the amplifier (4)which converts analog signals to digital sample values; a firstfrequency selective power estimation unit (15, 16) for determiningsignal level at a lower test frequency (f_(a)) within a frequency rangefrom 25 to 35 Hz; a second frequency selective power estimation unit(17) for determining the signal level at a higher test frequency (f_(b))with a frequency range from 55 to 65 Hz; a first and second peakdetector (42, 56) for monitoring said signal levels fluctuating in timeand estimated by said first and second power estimation units (15, 16,17) and establishing occurrence of local peaks of said signal levels; anidentification unit for identifying first and second heart sounds basedupon the time and the sequence of said local peaks and for computingfetal heart rate according to the method of claim 1; a storage means (8)for storing measured fetal heart rate values; and a communication port(10) for making said stored fetal heart rate values accessible forfurther evaluation.
 11. The apparatus as claimed in claim 10, whereinsaid peak detectors (42, 56) include comparators (43, 44, 45, 46)connected to parallel outputs of said fourth and sixth register files(41, 55); and an AND gate (47) connected to outputs of said comparators(43, 44, 45 and 46) indicating the local peak of said power level(P_(a), P_(b)).
 12. The apparatus as claimed in claim 10, wherein saidsound unit (5) includes a switch (62) adapted to turn on and offtransponded and audible heart sounds.
 13. The apparatus as claimed inclaim 10, wherein said serial communication port (10) is adapted to beconnected to a line printer.
 14. The apparatus as claimed in claim 10,wherein the acoustic sensor (1) includes a hollow body (27) and anelectroacoustic converter (24), located inside said hollow body (27) andhaving a sensitive membrane; wherein said hollow body (27) ispartitioned into a first, a second, and a third chamber (19, 20, 25);said first chamber (19) being open at one side, and coupled to saidsecond chamber (20) via an opening (21) formed in the partition wallbetween said first and second chambers (19 and 20), and having a firstboring (23) on its sidewall communicating with the atmosphere; saidsecond chamber (20) having an upper partition wall formed by a sensitivemembrane of an electroacoustic converter (24); and said third chamber(19) communicating with the atmosphere through a second boring (26)formed in the sidewall of the hollow body (27).
 15. The apparatus asclaimed in claim 14, wherein said electroacoustic converter (24) is anelectrodynamic microphone having an elastic membrane.
 16. The apparatusas claimed in claim 15, wherein an elastic ring (22) is provided at theperiphery of said open side of said first chamber (19).
 17. Theapparatus as claimed in claim 10, further comprising a tocodynameter (9)for continuous measurement of maternal womb contractions.
 18. Theapparatus as claimed in claim 10, wherein said analog unit (2) includesan active filter (3) connected to said sensor (1), said amplifier (4)being connected to said active filter (3) and a sound unit (5) which isconnected to said amplifier (4), the apparatus thereby amplifying,filtering and frequency transponding the input signal.
 19. The apparatusas claimed in claim 10, wherein said analog to digital converter (61) isincluded in a microcontroller, said microcontroller (6) being adapted toevaluate data and control peripheral units.
 20. The apparatus as claimedin claim 19, further comparing a standardized modem adapted to effectdata transfer for telemetric measurements and for communicating with themicrocontroller (6).
 21. The apparatus as claimed in claim 19, whereinat least one of said first frequency selective power estimation unit(15, 16), said second frequency selective power estimation unit 17, saidfirst peak detector (42) and said second peak detector (56) is includedin the microcontroller (6).
 22. The apparatus as claimed in claim 19,wherein at least one of said first frequency selective power estimationunit (15, 16), said second frequency selective power estimation unit(17), said first peak detector (42), said second peak detector (56 ) isincluded in an application-specific processor (7) connected to themicrocontroller (6), said application specific processor (7) furthercomprising a gain control circuit (18) connected to the gain controlinput of said amplifier (4).
 23. The apparatus as claimed in claim 17,wherein said storage means (8) includes a non-volatile memory (8)connected to said microcontroller (6) for storing measured fetal heartrate data and addition data provided by said tocodynameter (9).
 24. Theapparatus as claimed claim 19, wherein said communication port (10) is aserial communication port connected to the microcontroller (6) forconnecting said apparatus to an additional computer via a standardserial communication line.
 25. The apparatus as claimed in claim 19,further comprising an input device (12) connected to the microcontroller(6) for indicating subjective feeling of fetal movement.
 26. Theapparatus as claimed in claim 19, further comprising a light emittingdiode (13, 14) connected to the microcontroller (6) for indicatingamplifier overdrive and extremely low input signal level.
 27. Theapparatus as claimed in claim 26, wherein at least one of said frequencyselective power estimation units (15, 16; 17) includes a digital filter(15) adapted to select frequencies within said frequency range and saidpower estimation unit is connected to the output of said digital filter(15).
 28. The apparatus as claimed in claim 27, wherein said digitalfilter (15) includes: a register file (28) having serial-in,parallel-out structure and containing actual input data and precedingdata to be considered on the course of digital filtering; a segmenteddecoder (29) consisting of decoder segments for separately decodingparallel outputs of said register file (28); a segmented look-up table(30) storing words to be designated separately by said segmented decoder(29) consisting of products (a_(k)d_(n−k)) where n is the number ofsampling, d_(n−k), is the (n−k)th previously sampled input data, a_(k)are filter coefficients with k being in the range of k=0 to M, where Mrepresents the order of the filter; a second register file (33) havingserial-in, parallel-out structure and containing preceding filtered dataused for actual filtering; a second segmented decoder (32) consisting ofdecoder segments for separately decoding parallel outputs of said secondregister file (33); a second segmented look-up table (31) storing wordsto be designated separately by said segmented decoder (32) consisting ofproducts (b_(k)y_(n−k)), where y_(n−k) are (n−k)th previous filtereddata, b_(k) are filter coefficients with k being in the range of k=1 toN, where N corresponds to the order of the filter; a summator (24) forconsecutively reading out and summing each designed word of said tablesegments according to the following formula:${y_{n} = {{\sum\limits_{k = 0}^{M}{a_{k}d_{n - k}}} + {\sum\limits_{k = 1}^{N}{b_{k}y_{n - k}}}}},$

wherein y_(n) is the actual filtered data, and the meaning of otherdesignations are the same as above.
 29. The apparatus as claimed inclaim 21, wherein said power estimator (16) comprises: a decoder (35)for decoding filtered data as an address; a look-up table (36) addressedby said decoder (35) for containing squared values of said addresses; asummator (37) connected to said look-up table (36) for summing valuesconsecutively rad out of said table (36); a scanner (38) controllingrepetitive and consecutive addressing and summing said squared values bysaid summator (37); a third register file (39) connected to saidsummator (37) and having serial-in, parallel-out structure andcontaining new summed data in its upper register and the oldest data inits lowest register; a replacer (40) having a first input connected tothe upper register of said third register file (39), a second inputconnected to the lower register of the register file, and a third inputconnected to a fourth register file (41), said replacer (40) beingadapted for adding actual values at its first input to and subtractingactual values at its second input from the value received from saidfourth register file (41); the fourth register file (41) being connectedto an output of said replacer (40) and having a serial-in, parallel-outstructure storing calculated power values and shifting its contents downstep by step and having parallel outputs, the first of them connected tosaid third input of said replacer (40) and all inputs being connected toa peak detector (42) adapted to transfer contents of saidmicrocontroller (6).
 30. The apparatus as claimed in claim 29, whereinsaid second frequency selective power estimator unit (17) includes: afifth register file (48) having a serial-in, parallel-out structure andstoring input data (d_(i)) measured in a time window; a decoder (49) fordecoding data (d_(i)) stored in the fifth register file (48) anddesignating corresponding words of a second look-up table (50)containing products according to the following formulae: _(i) =d_(i)*sin(2πf _(b) i/n _(wb))*Ba(i); _(i) ≦d _(i)*cos(2πf _(b) i/n_(wb))*Ba(i); where d_(i) is the ith data sampled and digitized from theunfiltered signal, n_(wb) is the size of window in number of sampletimes, Ba(i) stands for the triangular correction function of a Bartlettwindow; a third look-up table (53) connected to summators (51) and (52)and containing squared values as words having a length equal to the wordlength of said summators (51,52); one of said summators (51) adapted forsumming words of said second look-up table (50) designated by saiddecoder (49) to provide an intermediate sum${S_{v} = {\sum\limits_{i = 0}^{n_{wb}}V_{i}}};$

where S_(v) is an intermediate sum and squaring this sum by using thethird look-up table (53) for providing a squared intermediate sum ofS_(v) ²; the other summator (52) adapted for summing words of saidsecond look-up table (50) designated by said decoder to provide a secondintermediate sum ${S_{w} = {\sum\limits_{i = 0}^{n_{wb}}W_{i}}};$

and for squaring this sum by using said third look-up table (53) forproviding squared intermediate sum of S_(w) ²; an adder (54) for addingsquared intermediate sums for providing a power level at a second testfrequency (f_(b)): P _(b)=(S _(v) ² +S _(w) ²)/n _(wb), where P_(b) isthe power in the actual time window for the frequency (f_(b)); a sixthregister file (55) having a serial-in parallel-out structure and storingthe last five sums of adder (54); and a second peak detector (56) forindicating peak values of said power (P_(b)).
 31. The apparatus asclaimed in claim 29, wherein said decoders (29, 32) and summator (34)are adapted to be turned in sleep mode to reduce power consumption whenno heartbeat is expected.